Ion mobility spectrometry is widely used as a simple low cost highly sensitive chemical analysis technique in trace explosives and narcotics screening applications by separating and identifying ionized molecules in the gas phase based on mobility of the ionized molecules. An ion mobility spectrometer (IMS) uses an ionization source, a drift tube, and an ion collector to detect the ionized molecules. The drift tube of an IMS is provided by a stack of rings on which a voltage schedule is applied in order to establish an axial field to move the ions from the ionization source to the detector. The detector consists of an ion collector (solid Faraday plate) and an amplifier stage converting incoming ion charge to a voltage that can be digitized for further processing.
FIG. 1 illustrates an issue with a conventional IMS 50 relating to the presence of radial variations in the electric field in the drift tube. The electric field lines are shown in FIG. 1. The drift field becomes progressively less homogeneous according to a distance from a main axis (central axis) of the drift tube. Any inhomogeneity in the electric field will cause a drift delay relative to an ion which experiences a perfectly uniform field. Field equipotential values are relatively straight near the main axis of the tube but become more distorted as the distance from the main axis increases. As a result, ions of identical mobility values moving near the edge of the drift tube will have a drift time different from that of ions moving near the main axis. This causes broadening of IMS peaks and therefore loss of resolving power. The greater the inhomogeneity in the electric field, the greater the difference in drift time and therefore the broader the IMS peak. Field distortions are also observed near the detector.
The distortion may be addressed by using a detector plate that is significantly smaller in diameter than the diameter of the drift tube. Although this improves resolving power of the IMS, using a relatively small diameter detector reduces sensitivity of the IMS by collecting fewer ions. The diameter of the collector plate for an IMS is a trade-off between resolution and sensitivity; the collector plate is small enough to collect only ions near the main axis but large enough to collect as many ions as possible in order to achieve good sensitivity.
In addition, for stand-alone IMS instruments, a charge is induced in the collector by the cloud of ions near the collector. The charge, when integrated over time, corresponds to a so called mirror current. The mirror current is undesirable because it leads to reduced resolving power by artificially broadening the detected width of the ion signals. The mirror current in an IMS may be addressed using an aperture grid (AG), which is a metal mesh placed at a distance 0.3 to 3 mm from the collector and biased with a voltage to act as an electrical shield. The AG prevents induced current flow in the detector caused by the approach of the cloud of ions. However, while the AG provides a solution to the mirror current problem, use of the AG reduces instrument sensitivity because many ions are annihilated when striking the wires of the mesh. In order to minimize ions losses, the mesh may be designed for high transparency using very thin wire. This makes the mesh relatively fragile and subject to distortions and damage due to vibrations and shock encountered by IMS instruments used in the field.
Accordingly, it is desirable to provide advantageous and efficient techniques that address the relatively small diameter used for the collector plate as well as eliminating the need for an aperture grid.